Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—I. Optimisation of barrier structure

In a number of metal matrix composite systems, there is a need to combat interfacial chemical attack and other forms of microstructural degradation. There is interest in producing thin surface coatings on (fibrous) reinforcements, to act as diffusion barrier layers or to otherwise modify the surface characteristics. An assessment of relevant factors is presented in terms of thermodynamic, kinetic and mechanical considerations. Possible advantages of a “duplex”, metal/metal oxide double layer, are examined with particular reference to a titanium matrix. Two procedures are described for the production of such barriers on SiC monofilaments using sputter deposition, one a batch process and the other involving continuous production. The effect of deposition conditions is considered in terms of optimising barrier structure.     

Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—II. System stress state

A potential advantage of sputter deposition as a process for producing protective coatings is that there is a degree of control over the stress state of the deposit. The factors affecting this are briefly discussed. Some experimental data are then presented for X-ray diffraction by {103} planes in a yttrium coating on SiC monofilaments. In order to interpret these data, it was necessary to set up a model describing the expected contributions of radial and hoop strains to the observed spectra. The stress state could then be deduced. Combination of these results with predictions from a stress analysis model allowed the differential thermal contraction stresses to be subtracted and the stress introduced during the deposition process to be estimated. An equal biaxial stress of about -800 MPa was obtained in this way. for a coating produced under conditions designed to favour a large compressive stress. This is consistent with observations that these coatings exhibit good mechanical stability.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—I. Haz temperature and strain rate distribution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part I the different components of the model are outlined and analytical solutions presented which provide quantitative information about the HAZ temperature distribution for a wide range of operational conditions. Moreover, a general procedure for modelling the HAZ strain rate distribution has been developed by introducing a series of kinematically admissible velocity equations which describe the material flow fields in the radial, the rotational, and the axial direction, respectively. Calculations performed for both types of materials show that the effective strain rate may exceed 1000 s⁻¹ in positions close to the contact section due to the high rotational velocities involved. Application of the model for evaluation of the response of AlMgSi alloys and AlSiC metal matrix composites to the imposed heating and plastic deformation is described in an accompanying paper (Part II).     

Densification and structure formation in the sintering of composite materials based on nitrides of the IV–V group transition metals. III. Sintering of composite materials in the (VN, TaN)—Cr systems

An investigation was made of densification and structure formation in composite materials in the systems (VN, TaN)—Cr during sintering in argon. It was shown that the shrinkage of these materials during liquidphase sintering is insufficient to provide dense composites (residual porosity was 35–40%). This is attributed to the low thermodynamic stability of VN and TaN, and the rapid evolution from these of nitrogen which accumulates in closed pores. Processes of heterodiffusion and alloy formation also have a negative effect on densification. Exchange reactions between chromium and the nitride-forming metals lead to the formation of a large quantities of intermetallics which embrittle the composite materials. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 13–18, January–February, 1999.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—III. The transformation between the high temperature phase and the 2H martensite

The stability between the high temperature β phase and the 2H martensite is analyzed in CuZnAl single crystals of a high electron concentration around 1.52. The results are compared with those deduced from previous reports, and the equilibrium temperatures and entropy differences are determined. A phenomenological model for the stress induced transformation explains well the observed crystallography.     

Phase transformations in the (Ti,Al)3 Nb section of the TiAlNb system—I. Microstructural predictions based on a subgroup relation between phases

Possible paths for the constant composition coherent transformation of b.c.c. or B2 high temperature phases to low temperature h.c.p. or orthorhombic phases in the TiAlNb system are analyzed using a sequence of crystallographic structural relationships developed from subgroup symmetry relations. Symmetry elements lost in each step of the sequence determine the possibilities for variants of the low symmetry phase and domains that can be present in the microstructure. The orientation of interdomain interfaces is determined by requiring the existence of a strain-free interface between the domains. Polydomain structures are also determined that minimize elastic energy. Microstructural predictions are made for comparison to experimental results given in Part II.